First make sure that you have used git-lfs to download the dolfin repository,as the xml files are not stored in the repo, as mentioned here: L-shape mesh generation with right element - #10 by Linda_Alzaben
To be able to plot the solution, you need to either save it to png import matplotlib.pyplot as plt; plt.savefig("file.png") instead of plt.show() or use Paraview to open output pvd or xdmf files.
In general, I would suggest you have a look at: 1. Quickstart — FEniCS Containers 1.0 documentation